The present invention relates generally to satellite communication systems, and more particularly to a method and system for alerting mobile terminals in a high-attenuation propagation environment within a mobile satellite communication network.
Mobile satellite communication systems generally comprise at least one orbiting satellite, at least one fixed ground terminal such as a gateway, and a plurality of mobile terminals. The gateway is connected to a ground-based public-switched telephone network (PSTN). The mobile terminals communicate with the gateway or other mobile terminals via the satellite over communication channels using time division multiple access (TDMA).
Either mobile users or PSTN users can initiate calls to any mobile user subscribed to the system (the "mobile-terminated call"). The mobile-terminated call is forwarded to its destination in similar fashion to forwarding within terrestrial telephone networks. A typical call will pass through mobile switches within gateways, satellite relays, and inter-gateway links until the call reaches the mobile switch to which the mobile intended to be called (the "called party") was last registered. The mobile switch within the gateway then attempts to page the called party using signalling control channels broadcast over the satellite coverage area. When the called mobile terminal detects the page and attempts to answer the call, the mobile switch allows the call to proceed.
Mobile satellite communication systems are preferred in rural or remote areas because the coverage area of the system tends to be larger than for a terrestrial system. In general, mobile terminal users can have a direct line-of-sight path from anywhere within the coverage region to the satellite with equal ease. In this regard, there are no geographical constraints on the location and size of the coverage area.
In mobile satellite communication systems, mobile terminals typically employ a low gain omnidirectional antenna of less than 6 dB gain. The low gain antenna collects the signal sent by an orbiting satellite's spot beam, including the direct line-of-sight components of the signal and the specular ground reflection components near the terminal. The antenna also collects multipath reflection components of the direct signal from taller stationary objects such as trees, mountains, and buildings.
These reflection components can combine destructively when collected and result in attenuation or fading of the signal. Moreover, more severe signal fading or attenuation may occur if the line-of-sight path between the mobile terminal and the orbiting satellite is blocked by a building or other object. This effect is called "shadowing."
As a result of these shadowing and reflective factors, a mobile terminal might not receive a paging signal sent by a gateway. Thus, the called party may fail to answer a mobile-terminated call when the user is inside a metal-framed building, underground, or otherwise experiencing severe signal fading or attenuation. The shadowed user has no way of knowing he or she is being called. These factors contribute to the low success rate (approximately 40%) of conventional mobile terminated calls.
Therefore, there is a need for an alerting system and method to allow a caller in a mobile satellite communication network to alert a mobile terminal user that he or she is being called when the user is in a heavily shadowed area.
Furthermore, there is also a need to integrate an alerting system capable of reaching highly attenuated mobile terminals using a conventional paging system within a mobile satellite communication network.